Unless specifically stated in a written contract, Arthur Harrison grants
no licence or permission to any party for the use of the information contained
in this document for the manufacture and sale of goods, nor any other commercial
utilization. Refer licensing inquiries to:
diy@harrisoninstruments.com.

Arthur Harrison assumes no liability for any damages, direct, or consequential,
which may arise from the dissemination, application, or misapplication of
the content contained in this site. The User of the information provided
in this site assumes all responsibility for any damages, direct or consequential,
which may arise from its use. Arthur Harrison retains the right to alter
the content within this site at any time without notice.

I designed the "Minimum Theremin" with the objectives of simplicity and economy,
and in response to frequent requests for an instrument that can be built
with readily-available parts. This is a pitch-only instrument that demonstrates
the basic theremin concept, perfect for science fair projects or as a coffee
table conversation piece. Although it doesn't have the qualities of more
expensive and complex theremins, it is very inexpensive, relatively easy
to make, and sufficiently sensitive and stable for playing a melody. A suggested
lay-out is included for perforated-board construction, however, any number
of alternative methods may be used to build the Minimum Theremin.

The 2006 version of the Minimum Theremin, described here, supersedes an earlier
version described in the original, 1999 article. This version contains changes
that permit the use of U1 and U2 integrated circuits from a wider variety
of manufacturers. Builders of the original Minimum Theremin may easily update
their circuit to the latest version.
Click here for instructions.

Two identical, commonly-available integrated circuits known as "hex inverters"
are used for the theremin's primary functions. These are CMOS (Complimentary
Symmetry Metal Oxide Semiconductor) devices, typically used in digital circuits
to perform a logic function called "inversion." Each IC contains six identical
sections; thus the term "hex inverter."

U1 sections A and B form the theremin's variable oscillator that operates
in a frequency range around 73kHz. The antenna forms one-half of a variable
capacitor that is part of this oscillator's frequency-determining network,
and the player's hand forms the other half. As the distance between the hand
and the antenna varies, so does the capacitance and therefore the oscillator's
frequency. U1 section C buffers the variable oscillator's output to provide
impedance isolation from the rest of the circuit.

U2 sections A and B comprise the theremin's local oscillator that is adjusted
with PITCH NULL potentiometer RV2 and PITCH NULL CALIBRATION potentiometer
RV1. RV1 is a miniature "trimmer" type, mounted on the circuit board, used
to calibrate the local oscillator's frequency range. With RV1 properly adjusted,
the local oscillator's frequency will equal the variable oscillator's frequency
with the hand furthest away from the antenna. Under these conditions, the
phase relationship of the two oscillators will be constant due to their small,
but finite capacitive coupling, so no audible tone will be produced.

The inverters' propagation delays and output impedances are supply-dependent.
Accordingly, PITCH NULL potentiometer RV2 affects the local oscillator's
frequency by varying its supply voltage. This method of adjustment permits
RV2 to be located at any convenient distance from the circuit and antenna,
since frequency variations resulting from stray capacitance between the
potentiometer's slider and ground are decoupled by capacitor C8. In use,
RV2 is adjusted by the player so that the theremin is silent with the hand
furthest away from the antenna, and produces the lowest tone when the hand
is at the maximum playing distance. U2 section C buffers the variable
oscillator's output to provide impedance isolation from the rest of the circuit.
The three remaining U2 sections are not used, so their inputs are grounded.

Diodes CR2 and CR3, and resistor R3 comprise a logic "OR" gate which is the
theremin's mixer. The mixer "heterodynes" the two oscillator signals, producing
sum and difference terms of their fundamental frequencies and harmonics.
U1 section F buffers the mixer's output. C4 is a DC blocking capacitor that
couples the mixed signal to a low-pass filter consisting of R4 and C5. This
filter reduces the inaudible heterodyne sum products, leaving the audible
difference, or "beat frequency" product. The beat frequency signal is applied
to an amplifier comprised of U1 sections D and E, and gain-determining resistors
R5 and R6. Capacitor C6 provides a second low-pass filtering section to further
attenuate the inaudible heterodyne products.

The following figures of the logic "OR" gate and related waveforms illustrate
the mixer operation. A 64kHz squarewave from the variable oscillator is applied
to input "A", and a 76kHz squarewave from the local oscillator is applied
to input "B." Waveform "C" is the mixer's output. In this example, the frequency
of each input waveform is above the hearing range, but the output waveform
is a pulse train containing an audible frequency related to their difference.
This is evident in the periodicy of waveform "C's" pulse clusters, that for
this example of frequency inputs, appear at an audible beat frequency of
12kHz.

Since the theremin's oscillators have some sensitivity to power supply
variations, VR1, a low-dropout voltage regulator IC, is used to furnish a
steady 5V to the circuit. The circuit uses less than 2mA of current, so a
nine-volt alkaline battery will provide many days of operation. Rectifier
CR1 protects the circuit from accidental battery reversal, and R12 prevents
excessive current from causing CR1 and the battery to heat under such a
condition. Since the circuit draws less than 2mA, the voltage drop across
R12 is less than 200mV, causing a relatively insignificant loss in battery
life. The instrument will operate with a battery as low as 5.5V.

In the prototype, the output at J1 is 2.75V peak-to-peak at 100Hz, decreasing
to about 450mV peak-to-peak at 1000Hz. The theremin's useful pitch range
is about 200Hz to 1600Hz, or three octaves, for a corresponding hand distance
of about eighteen inches to one inch from the plate antenna. The inherently
low "Q" characteristic of the oscillators establish a limit for the lower
audible frequency, below which, electromagnetic interference from 60Hz power
lines and appliances, combined with stray-capacitive coupling between the
oscillators, compromise the output fidelity. These conditions improve if
the instrument is far from interfering sources. The values of the oscillators'
frequency-determining components may be adjusted empirically to obtain the
best compromise of sensing range and fidelity. For example, as C3 is increased,
interference effects will diminish at the expense of decreased sensing range.

Regarding the difference between this version of the Minimum Theremin and
the earlier version: This version's mixer, comprised of a diode "OR"
gate followed by a buffer, produces a heterodyne signal that is a full-amplitude
(zero to +5V) pulse train with fast transitions, resulting in a tone quality
which has a greater proportion of harmonics, and a well-defined output amplitude.
The earlier version utilizes an uncharacterized CMOS inverter parameter that
varies from one IC to another, generally producing a more fundamental tone,
but not as consistent in amplitude nor harmonic content.

The Minimum Theremin may be connected to an amplifier via mono OUTPUT jack
J1. It also has sufficient output power to drive efficient, high-fidelity
headphones. For use with stereo headphones, the user may wish to replace
J1 with a stereo type that has its "tip" and "ring" contacts connected together.

NOTICE: Do not play this instrument at a high volume, especially
when using headphones. Use headphones that have a built-in volume control,
and adjust the volume control for a comfortable level. Hearing experts advise
against the continuous, extended use of headphones.

For a theremin to operate properly, sufficient capacitive coupling must exist
between the player's body and the instrument. Although not obvious, a player
is sufficiently "coupled" to earth via his or her surface area, which presents
a large capacitance to earth's ground through "free space." When the theremin
is connected to an amplifier, it too, is grounded via an electrical connection
and/or capacitance between the amplifier circuitry and earth. This "free
space" coupling, along with the amplifier's connection to ground, provide
the desired common connection between the player's body and the instrument.

When used with headphones and not connected to external equipment, there
may not be sufficient capacitive coupling between the instrument and player.
This may be remedied by connecting the circuit's ground (battery negative
terminal) to earth or a nearby metal object. A metal microphone stand, when
used to mount the theremin, is suitable for this purpose. If this is not
practical, grasping a few coils the headphone cord will provide sufficient
coupling between the player and the circuit's ground.

Temperature performance is quite excellent for this theremin; warm-up drift
is negligible due to the conservative power levels in the oscillator ICs,
as well as first-order drift cancellation between the identically-configured
oscillator circuits.

A kit of the major components for building a 2006 Minimum Theremin is available
from Harrison Instruments, Inc. The kit includes a blank printed circuit
board which greatly eases construction.
Click
here for ordering information. The blank board, alone, is also available
from Harrison Instruments as part number 101-PCB (item 13 in the table).

If you prefer to construct the 2006 Minimum Theremin without the printed
circuit kit, continue reading this page.

The lay-out drawing and images below illustrate the circuit board as implemented
in the prototype. The construction method utilizes perforated board (Vector
64P44WE, cut to a size of 2.9" x 4.1"), and push-in terminals (Vector T68).
The T68 terminals have a fork to accommodate component leads on the top of
the board, and 3-level Wire-Wrap® posts to accommodate Wire-Wrap®
connections on the bottom of the board. (Note: The lay-out drawing is a
simplified representation that depicts both the component and wire locations
on the same plane.)

To ensure low impedance, flat braid is used for the ground bus, located on
the component side of the board. The bus is made of the braid material normally
used for desoldering. Note that the braid is placed against several of the
vertically-oriented terminals and held in place with a thin seam of solder.
When this technique is used, inspect each connection carefully to ensure
that the braid is securely bonded to each terminal. For best results, the
braid material should be thoroughly "tinned" (solder coated) prior to use.

Note that trimmer potentiometer RV1 is furnished with three short wire leads.
To mount RV1, gently bend each lead 180 degrees, so that they are each parallel
to the part's housing. Then, nest the part between the three vertically-oriented
terminals as shown in the lay-out, and solder each of the leads between the
two tines of their respective terminals.

If you choose an alternative lay-out, maximize the distance between the U1
and U2 circuits to minimize their mutual capacitance. Since the impedances
in this circuit are high and therefore affected by electromagnetic interference,
the connections between the ICs and their associated components should be
kept as direct as possible; avoid excessive wire lengths between the terminals.

With regard to component selection, U1 and U2 should be from the same
manufacturer and production lot. This will help ensure that they will have
similar oscillation frequencies. Several manufactures make integrated circuits
suitable for the U1 and U2 positions. The following types will work:

Manufacturer

Part Number

Texas Instruments

CD4069UBE

On Semiconductor

MC14069UBCPG

ST Microelectronics

HCF4069UBEY

Toshiba

TC4069UBP

ICs including the designation "74_04," "HC," "HCT," "AHCT," "AC," or "ACT"
in their part number should be avoided in this circuit.

All the resistors are 1/4 watt, ±5 per-cent tolerance, carbon-film types.
±1 per-cent metal-film types with the nearest equivalent values may
be substituted. None of the capacitors are critical with the exception of
C3 and C9, which should be temperature-stable mica types with a ±5 per-cent
tolerance. Viable substitutes for C3 and C9 include polystyrene or NPO ceramic
types.

A note about soldering: The suggested lay-out requires considerable soldering
skills due to the compact arrangement. Novices should practice their soldering
with T-68 terminals, inexpensive components (such as resistors), and a separate
piece of perforated board prior to building the theremin. In particular,
be careful not to let a solder "bridge" form in the tiny gap between adjacent
terminals, such as the terminals for R5, R6, and C6.

The circuit board has a hole in each corner for 4-40 threaded standoffs,
used to mount the assembly in an enclosure such as the Keystone type 764
plastic box in the parts list. Orient the circuit board so that the adjustment
screw on RV1 can be conveniently accessed through a hole in the box. Arrange
the remaining items so that the battery is easily accessible, and the POWER
SWITCH, OUTPUT JACK, and PITCH NULL potentiometer are mounted in logical
positions.

There are no restrictions regarding the type of enclosure material, however,
for a metal enclosure, route the antenna wire away from surfaces, pass the
antenna wire through a hole fitted with a small rubber grommet, and attach
the antenna to the enclosure with an insulated support such as the wood post
in the Parts Table. In any case, the antenna should be mounted at least eight
inches away from the enclosure to prevent unwanted coupling between it and
other parts of the circuit.

If the theremin is equipped with an internal amplifier, enclosed in a wood
cabinet, and not grounded via external equipment, the effects of electromagnetic
interference may be reduced with a simple ground plane. The plane may be
made of any conductive material such as copper-clad circuit board or aluminum
foil. Its exact dimensions are not critical, although it should have an area
ranging in size from that of the of circuit board, and up to that of the
antenna. It is usually most convenient to mount the ground plane below the
circuit board, using the same mounting hardware that secures the board. Connect
the ground plane to any ground point on the circuit board with a short length
of wire. The ground plane's role in reducing interference effects is a result
of the common-mode cancellation it provides to the circuit ground with reference
to the antenna, and not a result of its shielding properties. However, when
mounted beneath the circuit board, such a plane will also be a benefit in
reducing the coupling between the U1 and U2 oscillators.

Although 1/16th inch-thick aluminum is indicated for the antenna, any conductive
material, such as aluminum foil or copper-clad circuit board, may be substituted.

See the "TEST ARRANGEMENT" drawing in the "Test and Calibration Procedure"
section for a pictorial representation of the theremin's major parts and
their interconnections.

Refer to the drawing below, "TEST ARRANGEMENT, MINIMUM THEREMIN." Arrange
the items as shown. The WOOD ANTENNA STAND should support the 8.5" x 5" ANTENNA
about ten inches from the work surface. (This distance is not critical.)
The J1 HEADPHONE JACK is Switchcraft part number 112B with its "tip" and
"ring" contacts connected together to accommodate typical stereo headphones.
It is advantageous to secure the CIRCUIT BOARD to the work surface so that
RV1 may be adjusted without having to hold the board in place with your hand,
because the added capacitance from your hand will affect the calibration.

Remove any objects such as wires or test equipment within two feet of the
CIRCUIT BOARD and ANTENNA.

Set the S1 POWER SWITCH to the "on" position. Connect stereo headphones to
the J1 HEADPHONE JACK.

Using a small screwdriver, set the RV1 PITCH NULL CALIBRATION POTENTIOMETER
to its extreme counterclockwise position. RV1 is a 15-turn potentiometer.
In some cases, end of the adjustment range is indicated by a faint audible
"click," but you may also simply turn the adjustment screw counterclockwise
15 full turns to ensure that it is at the counterclockwise extreme.

Set the RV2 PITCH NULL POTENTIOMETER to its middle position.

While listening to the headphones, and with the hand away from the ANTENNA,
slowly turn RV1 clockwise. Typically, no sound will be heard with RV1 in
its extreme counterclockwise position. As RV1 is rotated, a very high pitch
will become evident. The pitch will become successively lower with continued
rotation, until it stops abruptly. Stop turning RV1 at this point.

Remove the adjustment screwdriver from RV1. Note that the pitch might return;
this is a normal condition resulting from the capacitance change that occurs
with the screwdriver's removal.

Slowly adjust the RV2 PITCH NULL POTENTIOMETER just to the point where the
pitch stops. This point may occur either slightly clockwise or counterclockwise
of its center position.

Starting from a distance of about three feet, move your hand toward the ANTENNA.
Note that the pitch commences at its lowest frequency with your hand about
eighteen inches away, increasing as the distance shortens.

Here's an economical and effective way to package the Minimum Theremin. This
1999 version, constructed by the author, has a 6 1/2"-square aluminum antenna
supported by a 7"-long, rigid copper wire. Banana plugs soldered to each
end of the support wire plug into jacks on both the antenna and box top.
The power switch and pitch null potentiometer are mounted on the cover's
lower right area. The circuit board, speaker, and battery are mounted to
the inside surface of the front cover, and the ground plane is mounted to
the inside-bottom. There is space in the box to store the antenna and its
support.

The "Cigar Box" Minimum Theremin

Richard Bobrucki of Newbury, England constructed this beautiful version of
the Minimum Theremin in a wood case with an elliptical metal drawer handle
for the antenna, reminiscent of 1930s styling. For this theremin, about eight
inches of sensing distance was obtained, somewhat less than the nominal 18
inches specified. The reduction is likely a result of the relatively small
antenna geometry and its proximity to the circuit board. The theremin's cabinet
is approximately 8 inches square.

Jairo Moreno, an exceptional musician from Barcelona, Spain, adapted his
Minimum Theremin with a shoulder strap.

Jairo recorded this fine rendition of
"Malvarmo," his adaptation of Eduard
Artemiev's "Russian Theme" from the film "Urga." Malvarmo means "cold" in
Esparanto.

Renata Cortez Sica, a piano instructor from Araras, Brazil, constructed this
Minimum Theremin as one of her first electronics projects. See
Renata's
web page with more information about herself and her interests.

Dominik Bednarz, from Germany, constructed his Minimum Theremin in a cigar
box, with an audio amplifier based on the LM386 integrated circuit. The right
photo shows the sliding cover of the box, which can be adjusted to set the
resonance of the theremin's tone. Dominik also incorporated a photoresistive
cell in his design, which serves as proximity-controlled volume modulator.
The photoresistive cell is located in the matchbox, seen in the left photo.

Raffaello Bisso of Genova, Italy built this Minimum Theremin in a plastic
enclosure with a transparent lid and a disc antenna. He added an LED to provide
a visual indication of zero beat, and reports that the sensing distance is
quite adequate, despite the somewhat-reduced antenna area. Raffaello also
reported that his instrument has no AC noise nor unpleasant sounds, and a
clean output.

Bálint Kollár of Hungary built a Minimum Theremin in a small
plastic tool case. The red LEDs are for decoration, and the antenna is the
flat plate attached to the top. Bálint reports a 12 inch sensitivity
range, and was "absolutely amazed by the results."

Here is another version of the Minimum Theremin created by Bálint
Kollár. He calls it the "Mini Minimum," and it is likely one of the
smallest versions of the Minimum Theremin made to date. The image on the
right has a matchbox to show the truly miniature size of this instrument.
Bálint reports a sensing distance of 10 to 12 inches, and the ability
to obtain low-pitched notes, as well.

Felix Obée from Germany adapted this Cookie Monster puppet to incorporate
a Minimum Theremin. He has named it the "Quäkmonster," demonstrated
in this YouTube video.
The theremin's circuit board, an amplifier, and copper plate antenna are
in the puppet's tummy, and a small loudspeaker in the mouth produces a tone
when a hand is brought near. "Quäk," translated to English, means to
mewl (cry softly). The piece was displayed in an art exhibition in Frankfurt.
Felix reports, "I worked with various theremin circuits before this one and
I found it very reliable." More about Felix's work can be found on his
webpage.
The Cookie Monster was created by Jim Henson in the 1960s, and featured on
the children's television show "Sesame Street."

Sean Boyce constructed this 2006 Minimum Theremin using a cigar box enclosure,
a rod from a printer as the antenna mast, and a piece of copper-clad circuit
board for the antenna. Sean reports excellent sensitivity of up to four feet
with this instrument, with good player and theremin grounding.

Josh Hulbert built this elegant version of the 2006 Minimum Theremin in a
box made of beech and walnut. The metal on-off toggle switch is connected
to the theremin's ground, and the player touches the switch with one hand
while playing the instrument, to ensure good coupling. Josh reports a sensing
range of about 18 inches. and left RV2, the pitch zero control, on
the internal circuit board, for simplicity of appearance. Josh reports "It's
mostly a desk toy for my father, but he's happily picking out tunes."

Mike Tilley cleverly reused his
Minimum
Theremin kit's shipping container as an enclosure, and an aluminum soft
drink can attached with an alligator clip for his antenna. Mike reports great
results with his theremin; he obtains the entire audible range over a hand
distance of 15 inches.

Mark Venczel created his Minimum Theremin in a painted wood knife box. He
included the optional amplifier section with a small internal speaker, as
well as an output jack for connection to an external amplifier. Switches
with LED indicators control the power and the internal amplifier. The antenna
and copper antenna stand can be stored inside the box. Mark reports a sensing
distance of up to three feet, under optimum conditions in an interference-free
area.

Valerio from Italy created this unique Minimum Theremin in just a couple
of hours. He reports a rather short sensing distance of about four inches,
which is a result of the small antenna. Regardless, Valerio is leaning to
play some tunes with this instrument.

Marco built his Minimum Theremin in a cigar box, and added a volume control
potentiometer which is seen on the right side of the box. The antenna post
is mounted on a side panel, allowing the box to be opened while the theremin
is being played. A red LED on the circuit board indicates "power on."

Daniel from Sweden built his Minimum Theremin in a oval metal box. Using
the current from a 12V power supply, along with vinegar, salt and copper
braid, he created a galvanized patina on its surface. He used an LM386 integrated
circuit as an amplifier, with potentiometers for volume and overdrive, and
included a small speaker.

Alastair Rae from England constructed this excellent version of a Minimum
Theremin in a modern-style acrylic enclosure. The column of ten blue LEDs
are driven by a special circuit he devised. The LEDs flash sequentially at
a rate proportional to the theremin's output pitch. In addition, Alastair
included an LM386 audio amplifier IC and a half-watt speaker. The theremin
runs from an external 9VDC power source, and has a sensing range of about
six inches.

Rob Woostalker constructed his Minimum Theremin with a number additions,
including a LM386 amplifier IC, a waveshaping section using diodes, and an
optical volume control. He has a board-mounted pole antenna that can be
substituted with a plate antenna (not shown). Rob reports a sensing range
of about two feet with the plate antenna, and about one foot with the pole.

Mark Kipping from The Netherlands submitted this image of his Minimum Theremin.
He used a 7 inch diameter copper-clad circuit board as the antenna, and reports
that "...its stability and responsiveness are very good, considering the
circuit contains so little components." A small computer speaker with built-in
amplifier is used as the sound monitor.

Victor Perounkov from Yekaterinburg, Russia built his version of the
Minimum Theremin in a prisma-suprematist enclosure reminiscent of Kasimir
Malevich's cubo-futurist style. This was Victor's first electronics construction
experience, and he is very pleased with the results. The peripheral components
such as the knob and switch were taken from old Soviet electronics devices.
The foil face on the exterior serves as the antenna. The author is delighted
that this theremin has its home in Russia, the country of Leon Theremin's
original work in 1918.

Bill Colsher built his Minimum Theremin in a "Road Case" enclosure from Road
Rage Pro Gear(TM). The plate antenna can be seen in the background with an
RCA-type plug that fits into the jack on the top of the case. Bill has plans
to build a second Minimum Theremin in another Road Case enclosure, along
with a delay.

Dan Ally built his Minimum Theremin kit into an electrical box usually used
for electrical appliances. A grounded metal box and full-size antenna plate
exemplified in Dan's construction will ensure optimal sensing distance.

Kynar® is a registered trademark of Elf Atochem North America, Inc.
Teflon® is a registered trademark of E. I. du Pont de Nemours and
Company.
Wire-Wrap® is a registered trademark of Cooper Industries, Inc.
Destination ImagiNation® is a registered trademark of Destination
ImagiNation, Inc.